Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.

In a continuing effort to provide archival material for researchers
and for anyone seeking information on a particular radio restoration
project, this Radio Service Data Sheet for the Zenith Model 430/440
radio from a 1933 edition of Radio-Craft is being posted. An Internet
search will show that there are many people engaging in such activities.
Restoring my
Crosley Model 03BC console radio would have been more difficult
if not for others who have done similar work to
assist the 'community.'
I generally despise the phrase "giving back" because it is usually
uttered by people that really owe nothing to anyone, but somehow
feel obligated to do so or are conditioned to automatically say
such things. This is a case where I benefitted from somebody else's
work and there is an opportunity to return the favor.

Update: Mr. Steve of The Radio Fox website wrote to express thanks for having this information
posted. He owns both a
Zenith Model 430 and a Zenith Model 440(click thumbnails).

The newest product of Zenith Radio Corp. is the model 430 receiver;
equipped with the Zenith automatic tuning device, it is the model
440. The outstanding feature of this set is the "class AAA" system
of A.F. amplification used;

The condenser gang in this receiver is aligned at 1,500 kc.;
the oscillator padder is aligned at 600 kc.

Field coil No. 1 has a resistance of 1,600 ohms; field coil No.
2 has a resistance of 2,000 ohms for section A and 3,500 ohms for
section B. Choke Ch.2 has a resistance of 400 ohms.

There are no circuit changes or switches in this receiver for
the reception of police calls. The tuning dial, itself, actually
takes in the police frequencies since it covers a total range of
540 to 1,750 kc.

"Shadowgraph" tuning is effected by means of a vane fastened
to a meter movement connected in the plate-supply circuit to tubes
V1, V2, V3. The incoming signal causes this "meter" to move in the
usual manner, thus twisting the vane so that it either obstructs
the light and casts a wide shadow (off resonance), or permits the
light to pass, casting a shadow no wider than the thickness of the
vane (exact resonance of the tuned circuits).

Class AAA A.F. amplifier is the Zenith designation for a system
employing three power tubes, each of which are biased as class A
amplifiers, one of which is used to drive the other two in push-pull.
If the grids were overdriven with the usual type of coupling transformer,
having a high secondary impedance, between the driver and the output
tubes and the push-pull grids overdriven, extreme distortion would
take place. This difficulty is overcome by employing a special transformer
of step-down ratio having larger wire and much less resistance in
its secondary. This transformer is a very important part of the
system. With such an arrangement it is possible to drive the grids
of the power tubes positive without introducing distortion, because
of the driver being a power tube and capable of supplying the necessary
current on such peaks. The primary of this transformer has a higher
impedance than is normally used, which value is somewhat reduced
as the impedance changes in the secondary, when the power grids
are driven positive. Its impedance, however, at the lowest value,
still matches the driver plate circuit. It should be remembered
that in an ordinary class A amplifier distortion enters when the
grids are driven positive by virtue of the previous tube not supplying
sufficient power for grid excitation of the output tubes. In other
words, in class A, voltage on the power grids is available as long
as they do not go positive. As soon as they are driven past zero
in that direction the previous amplifier can no longer supply the
necessary power.

Summing up the above, the triple A amplifier is a combination
of both class A and class B. The exception being that the tubes
are biased as class A, but by virtue of the power delivered by the
driver and the special coupling transformer, sufficient power is
supplied for the power tube to draw grid current and still prevent
distortion as in the class B amplifier.

Many set analyzers will not accommodate the new tubes, consequently,
all voltage readings are to ground. Thus, for instance. the actual
voltage on the plates of the power tubes is 295 V.

The two reproducers are similar in construction but one is peaked
at 90 cycles and the other at 70 cycles. (This is done to cancel
the natural period of each reproducer.) The paralleled voice coils
result in four times the power obtainable from a single reproducer,
states the manufacturer.

An overall sensitivity of less than one-half microvolt-per-meter
is obtained with an undistorted power output of 15 watts. The total
line consumption at 115 V. is 125 W.

Posted March 24, 2015

Radio Service Data Sheets

These schematics, tuning instructions, and other data are reproduced from my collection
of vintage radio and electronics magazines. As back in the era, similar schematic and service
info was available for purchase from sources such as
SAMS Photofacts, but these printings
were a no-cost bonus for readers. There are 220 Radio Service Data Sheets as of
February 2, 2018.